[HotColdSplitting] Use TTI to inform outlining threshold

Using TargetTransformInfo allows the splitting pass to factor in the
code size cost of instructions as it decides whether or not outlining is
profitable.

This did not regress the overall amount of outlining seen on the handful
of internal frameworks I tested.

Thanks to Jun Bum Lim for suggesting this!

Differential Revision: https://reviews.llvm.org/D53835

llvm-svn: 346108